The invention relates to synchronous machines having the exciter armature windings, rectifier and main field winding on the rotor and more particularly, to circuitry for reducing the effect of transient voltages and currents in the main field winding and rectifier.
In synchronous machines having the main field windings, the exciter armature windings and the rectifier for supplying the main field with a DC current all located on the rotor, the relatively long transient recovery time for the field current when the load is removed from the main armature has posed a very significant problem. In addition, the necessity for protecting the rectifier and associated rotor components from the relatively high voltages generated in the main field when the synchronous machine, operated as a motor, is started from rest has similarly posed a very significant problem. Normally, when the machine is operating in a generating mode, a voltage regulator adjusts the current through the exciter field so as to provide the desired output voltage. Specifically, the exciter field current results in the generation in the exciter armature windings of a current which is rectified by the rectifier and applied to the main field winding to generate the desired generator output voltage. However, when the generator load is removed from the main armature, a portion of the main field current continues to flow for an appreciable amount of time due to the high inductance of the main field coil and the low resistance of the rectifier diodes. Thus, the generator output voltage transient on load removal tends to be relatively high in amplitude and long in duration. A similar problem is encountered when the synchronous machine is used as an induction motor utilizing the amortisseur winding as a squirrel cage. As the main armature windings are excited with a polyphase AC current, a rotating magnetic field is established which periodically sweeps by the single phase main field winding. The flux sweeping by the stationary or slowly moving rotor tends to generate potentially destructive high voltages in the field winding. Also, since the induced voltage is half-wave rectified by the rectifier, the resulting induced current in the main field tends to increase losses and produces torque cusps in the induction motor torque curve. One solution for the induction motor start-up problem has been to short-circuit the main field winding utilizing a centrifugal switch during start-up. The case of centrifugal switches for this purpose has proved to be, in some respects, unsatisfactory, since they tend to add mechanical complexity to the generator and additionally have moving parts and sliding contacts which provide maintenance problems and in some cases tend to be unreliable.